1. Field of the Invention
The present invention relates to a wave-shaping apparatus and a reproduction signal processing apparatus including the same, and specifically a wave-shaping apparatus for optimally performing digitization or A/D (analog/digital) conversion of a reproduction signal obtained by an optical pickup circuit, and a reproduction signal processing apparatus including the same.
2. Description of the Related Art
Recently, optical disks are being used in various conditions and stored in various manners. Under the circumstances, surfaces of optical disks are often scratched or exposed to dust. In order to read information from an optical disk having dust or scratches, a high precision signal determination is required.
Especially, an optical disk having information at a high density such as a DVD (digital versatile disk) has an inferior SNR (signal/noise ratio) at the shortest pit portion. Accordingly, it is required to minimize errors when a signal is digitized.
Information recording to a CD (compact disk) is performed by EFM (eight to fourteen modulation), and information recording to a DVD is performed by 8-16 modulation. These modulation systems make the spectrum of recording pattern to the disk substantially DC component-free. In order to make use of such characteristics of these modulation systems, a reproduction signal is usually digitized by a digitizing circuit for controlling the slicing level of the signal by performing negative feedback control, so that the duty ratio of the digitized signals is 50:50.
With reference to FIG. 14A, a digitizing circuit 1300 will be described.
An optical pickup circuit 2 outputs a reproduction signal RS obtained from an optical disk 1. The reproduction signal RS is capacitance-coupled by a capacitor C and supplied with a prescribed bias voltage of, for example, VCC/2. The resultant signal is input to a negative (xe2x88x92) input terminal 110A of a comparator 110 and compared with a slicing level which is input to a positive (+) input terminal 110B of the comparator 110. Thus, a digitized signal DG1 is output from the comparator 110.
A charge pump 101 is driven, i.e., the potential of a charge capacitor 102 is increased or decreased, in accordance with the polarity of the digitized signal DG1 output from the comparator 110. A charge voltage stored in the charge capacitor 102 is current-amplified by a buffer 103 and ripple-removed by a low pass filter 104. Then, the resultant signal is input to the positive input terminal 110B of the comparator 110.
For example, when the potential of the negative input terminal 110A is higher than the potential of the positive input terminal 110B, the output level from the comparator 110 is 0. Then, a charge pump 101A is turned on to raise the potential of the charge capacitor 102. Thus, the potential of the positive input terminal 110B of the comparator 110 is raised. In this manner, negative feedback control is performed so that the difference between the potential of the positive input terminal 110B and the potential of the negative input terminal 110A becomes 0.
When the potential of the negative input terminal 110A is lower than the potential of the positive input terminal 110B, a charge pump 101B is turned on to reduce the potential of the charge capacitor 102. Thus, the potential of the positive input terminal 110B of the comparator 110 is lowered. In this manner, negative feedback control is performed so that the difference between the potential of the positive input terminal 110B and the potential of the negative input terminal 110A becomes 0.
FIG. 14B is a waveform diagram of a high frequency reproduction signal RS. When such a high frequency reproduction signal RS in input to the digitizing circuit 1300, a negative feedback digitizing slicing level SL is controlled so that the duty ratio of the xe2x80x9c0xe2x80x9d level L0 and the xe2x80x9c1xe2x80x9d level of the digitized signal DG1 output from the digitizing circuit 1300 becomes 50:50.
A response ability of the negative feedback control is determined by the driving current value of the charge pump 101, the capacitance of the charge capacitor 102, and the time constant of the low pass filter 104.
Recently, electric circuits are more and more digitized. An analog reproduction signal from an optical disk is processed by an A/D converter with multiple bits, and then processed by a digital signal processing circuit for reproducing the signal entirely by digital processing. The digital signal processing circuit is generally referred to as a xe2x80x9cdigital read channelxe2x80x9d. Use of a digital read channel realizes stable operations by suppressing dispersion among circuits and reduces an error rate by digital signal processing such as, for example, PRML (partial response/most likelihood).
When an optical disk has defects such as dust or scratches, laser light from an optical pickup circuit is blocked by the dust or scratches. Accordingly, the level of the reproduction signal is significantly fluctuated in terms of both direct current or alternate current. In order to accurately digitize the reproduction signal despite such changes, the digitizing circuit 1300 needs to have a higher response ability so as to follow the changes.
However, the spectrum of recording pattern to the optical disk is not completely DC component-free around a defect change frequency (several kilohertz or less). Thus, the cut-off frequency of the low pass filter 104 needs to be raised to improve the response ability of the digitizing circuit 1300. In such a case, a DC fluctuation component of the reproduction signal is mixed into a negative feedback signal, i.e., the signal input to the positive input terminal 110B as an external disturbance. This prevents accurate digitization, and a data slicing error occurs and generation of jitters is increased.
FIG. 15 shows a circuit designed for alleviating the data slicing error. The reproduction signal RS is passed through a high pass filter 105, before being input to the digitizing circuit 1300, to remove a low frequency band fluctuation component, so that the reproduction signal RS has a line of symmetry between the upper half and the lower half even when the signal level is fluctuated by defects. In this manner, the control load performed by the digitizing circuit 1300 is alleviated and thus the data slicing error is alleviated.
However, such a circuit removes the low frequency band fluctuation component in a steady state, i.e., even when there is no defect. Therefore, a data slicing error occurs by the lack of information provided by the low frequency bank fluctuation component and the generation of jitters is increased.
FIG. 16 shows a signal level when a light beam passes through a defect. Since light reflected by the optical disk is not obtained, the level of the reproduction signal RS is lowered to a black level BL. This phenomenon is conspicuous in the case of dual layer DVDs. When performing A/D conversion of the reproduction signal RS, the dynamic range of A/D conversion usually needs to be maximized with respect to an amplitude A of a signal of normal reproduction from the viewpoint of SNR. However, when the signal level is reduced to the black level BL by the dropout, the reproduction signal RS exceeds the dynamic range. This adversely influences the digital signal processing.
According to one aspect of the invention, a wave-shaping apparatus includes a first detection circuit for detecting a reproduction signal reproduced from an optical disk by an optical pickup circuit using a first detection time constant to detect an upper envelope of the reproduction signal; a second detection circuit for detecting the reproduction signal using a second detection time constant to detect a lower envelope of the reproduction signal; an averaging circuit for putting a weight represented by a weighting coefficient to each of the upper envelope and the lower envelope to calculate an average value of each of the upper envelope and the lower envelope; and a subtraction circuit for subtracting a signal corresponding to the average value from the reproduction signal to output a wave-shaped signal.
In one embodiment of the invention, the wave-shaping apparatus further includes a low pass filter for smoothing the average value to output a smoothing signal. The subtraction circuit subtracts the smoothing signal from the reproduction signal.
In one embodiment of the invention, the weighting coefficient is 1:1.
In one embodiment of the invention, the weighting coefficient is determined based on an asymmetry quantity of the reproduction signal.
In one embodiment of the invention, the wave-shaping apparatus further includes an asymmetry detection circuit for detecting the asymmetry quantity.
In one embodiment of the invention, at least one of the first detection time constant and the second detection time constant is determined to be substantially in proportion to a diameter of a light spot on the optical disk.
In one embodiment of the invention, at least one of the first detection time constant and the second detection time constant is determined to be substantially in inverse proportion to a reproduction linear velocity of the optical disk.
In one embodiment of the invention, a cutoff frequency of the low pass filter is determined to be substantially in inverse proportion to a diameter of a light spot on the optical disk.
In one embodiment of the invention, a cutoff frequency of the low pass filter is determined to be substantially in proportion to a reproduction linear velocity of the optical disk.
In one embodiment of the invention, the wave-shaping apparatus further includes a dropout detection circuit for detecting a dropout indicating a decrease in an amplitude of the reproduction signal, wherein when the dropout is detected by the dropout detection circuit, at least one of the first detection time constant and the second detection time constant is set to be shorter than the time constant when no dropout is detected.
In one embodiment of the invention, the wave-shaping apparatus further includes a dropout detection circuit for detecting a dropout indicating a decrease in an amplitude of the reproduction signal, wherein when the dropout is detected by the dropout detection circuit, the cutoff frequency of the low pass filter is set to be higher than the time constant when no dropout is detected.
According to another aspect of the invention, a reproduction signal processing apparatus includes a wave-shaping apparatus which includes a first detection circuit for detecting a reproduction signal reproduced from an optical disk by an optical pickup circuit using a first detection time constant to detect an upper envelope of the reproduction signal, a second detection circuit for detecting the reproduction signal using a second detection time constant to detect a lower envelope of the reproduction signal, an averaging circuit for putting a weight represented by a weighting coefficient to each of the upper envelope and the lower envelope to calculate an average value of the upper envelope and the lower envelope, and a subtraction circuit for subtracting a signal corresponding to the average value from the reproduction signal to output a wave-shaped signal; and a digitizing circuit for digitizing the wave-shaped signal to output a digitized signal.
According to still another aspect of the invention, a reproduction signal processing apparatus includes a wave-shaping apparatus which includes a first detection circuit for detecting a reproduction signal reproduced from an optical disk by an optical pickup circuit using a first detection time constant to detect an upper envelope of the reproduction signal, a second detection circuit for detecting the reproduction signal using a second detection time constant to detect a lower envelope of the reproduction signal, an averaging circuit for putting a weight represented by a weighting coefficient to each of the upper envelope and the lower envelope to calculate an average value of the upper envelope and the lower envelope, and a subtraction circuit for subtracting a signal corresponding to the average value from the reproduction signal to output a wave-shaped signal; and an analog/digital converter for performing analog/digital conversion of the wave-shaped signal.
In one embodiment of the invention, the reproduction signal processing circuit further includes a digital signal processing circuit for performing digital signal processing of an output from the analog/digital converter to output reproduction data and a reproduction clock.
According to one aspect of the invention, the fluctuation of a reproduction signal caused when an optical disk has a defect is suppressed. Thus, the control load of a digitizing circuit is significantly alleviated. As a result, data slicing errors are significantly reduced without increasing the jitters in a steady-state reproduction.
According to another aspect of the invention, the control load of a digitizing circuit is significantly alleviated and data slicing errors are significantly reduced without increasing the jitters in a steady-state reproduction with a relatively simple structure.
According to still another aspect of the invention, the weighting coefficient provided to upper and lower envelopes used by an averaging circuit for calculating an average value thereof is determined to be substantially in proportion to an asymmetry quantity of the reproduction signal. Therefore, the control load of a digitizing circuit is significantly alleviated regardless of whether the writing conditions of data to the optical disk (e.g., the diameter of the light spot), and data slicing errors are significantly reduced without increasing the jitters in a steady-state reproduction.
According to still another aspect of the invention, wave-shaping is optimally performed based on the diameter of the light spot in the optical disk. Thus, data slicing errors are significantly reduced without increasing the jitters in a steady-state reproduction.
According to still another aspect of the invention, wave-shaping is optimally performed based on the reproduction linear velocity of the optical disk. Thus, data slicing errors are significantly reduced without increasing the jitters in a steady-state reproduction.
According to still another aspect of the invention, the wave-shaping performance when the optical disk has a defect is improved while and the generation of jitters when the optical disk has no defect is not deteriorated.
Thus, the invention described herein makes possible the advantage of providing (1) a wave-shaping apparatus for alleviating the load of a digitizing circuit and significantly decreasing the generation of jitters, and a reproduction signal processing apparatus including such a wave-shaping apparatus; (2) a wave-shaping apparatus for accurately digitizing a reproduction signal even when the level of the reproduction signal obtained from an optical disk significantly fluctuates by a defect on the optical disk as well as when the optical disk has no defect, and a reproduction signal processing apparatus including such a wave-shaping apparatus; (3) a wave-shaping apparatus for making maximum use of the dynamic range of an A/D converter, and a reproduction signal processing apparatus including such a wave-shaping apparatus; and (4) a wave-shaping apparatus for improving the stability of the digital signal processing circuit, and a reproduction signal processing apparatus including such a wave-shaping apparatus.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.